In reply to teflonpete:
When I was a teenager I went to Kenya with BSES. Since then I've led for BSES and am in the process of applying to lead for World Challenge.
I would agree with the consensus on this thread that it's a very worthwhile thing to do. I wouldn't worry too much about whether universities or employers look favourably on it (though I suspect the former do as it something different and the latter aren't bothered) but I would argue that the whole process is a great learning experience. When I was a teenager the trip instilled in me a love of the outdoors and travel that I still have today. It was also the first time I had been away for a significant period on my own (i.e. without parents, schoolfriends, siblings etc) which was great for encouraging independence. As a leader I can honestly say that watching the youngsters at the end of the trip and comparing them to the kids that turned up is one of the most rewarding things I have ever done.
Financially it's a big ask and it doesn't sit easy with me that the price tag essentially makes this a middle class endeavour (there are exceptions). I did a lot of fundraising for mine, my parents gave me £500 I think and I earnt / raised the rest. Others had the whole lot given to them. Although it would be great if it was more inclusive, I don't think those that can afford it should forego the opportunity out of a misplaced sense of guilt.
If nothing else, it may force your kid out of their comfort zone earlier than their peer group, they may learn something about themselves or about others, either good or bad. Even if they don't then a few weeks in the developing world, around those considerably less favourable than themselves, is infinitely more valuable than a car, a lads/girls holiday or an expensive 18th party in my opinion.
In summary: great experience, I would encourage my kids to attend if they showed an interest.